1. Field of the Invention
This invention relates to ethylene-vinyl alcohol (EVA) copolymer hollow fiber membranes and a method of producing the same. More particularly, it relates to novel EVA copolymer hollow fiber membranes presenting a three-layered structure comprising two layers consisting of bonded particles and a homogeneous layer lying therebetween and at least one active or skin layer on the surface of said membrane, and to a method of producing the same.
2. Description of the Prior Art
Selectively-permeable membranes, especially hollow fiber membranes, are coming into wide use in medical fields such as hemodialysis as well as in industrial fields such as in the ultrafiltration of various solutions.
EVA copolymer membranes which we have previously developed exhibit excellent biocompatibility as well as membrane performance characteristics such as water permeability and permeability to substances having medium molecular weights. Consequently, the utility of such membranes has been widely recognized in various fields.
Several EVA copolymer membranes have already been provided by the present inventors. Thus, U.S. Pat. No. 4,134,837, discloses an EVA copolymer membrane having a structure comprising particles which have particle sizes of 100 to 10,000 Angstroms and which are bonded to each other, said structure being recognizable throughout the membrane. The membrane is excellent as a dialysis membrane in hemodialysis. In copending U.S. patent application Ser. No. 962,962, filed Nov. 22, 1978, now U.S. Pat. No. 4,220,543, issued Sept. 2, 1980, there is disclosed an anisotropic membrane which is made of EVA copolymer compositions having different ethylene contents and contains cylindrical voids having a large average length as well as spherical voids having sizes ranging up to 20 microns. Said membrane, too, is excellent as a dialysis membrane. Further, in copending U.S. patent application Ser. No. 06/71,671, now U.S. Pat. No. 4,269,713, issued May 26, 1981, there is disclosed an anisotropic membrane which has a porous supporting layer containing vacuoles whose longitudinal lengths correspond to 20 to 99% of the membrane thickness. The membrane has a porosity of 60 to 90%. It is excellent as a filtration membrane for ultrafiltration.
As is mentioned above, membranes with different structures can be produced from EVA copolymers by varying production conditions. Therefore, EVA copolymers have been recognized as especially excellent materials for membranes.
As a result of further investigations, the present inventors have succeeded in producing a novel EVA copolymer hollow fiber membrane which is different in structure from those membranes described above.